Article surveillance systems that work on the principle of detecting the presence of a particular form of ferromagnetic material in a periodically changing low level magnetic field are well known. U.S. Pat. No. 4,553,136 illustrates a representative system of this type in which an article to be protected against unauthorized removal from a defined area, such as a store or library, has a marker attached to it that has in it a strip of low coercivity ferromagnetic material of high permeability. When the article, with the marker attached, is brought into an interrogation zone in which the low level periodically changing magnetic field is applied via a radiating antenna located at the edge of the zone, the presence of the strip modifies the pattern of the applied field, generating magnetic sub-fields at harmonics of the fundamental frequency of the applied field. These harmonic fields are picked up by a receiving antenna also located at the edge of the interrogation zone and fed to a receiver where they are detected and used to trigger an alarm to signal the unauthorized removal of the article. Other representative examples of ferromagnetic marker systems of this general type are found in U.S. Pat. Nos. 3,665,449; 3,747,086; 3,820,104 and 4,710,752.
There is a recognized need for surveillance systems to provide corresponding security protection for computer disk magnetic media. With the proliferation in business and government of personal computers and the ready portability and easy concealability of the miniature data disks used by these computers it is important to provide article surveillance sensor systems for computer data storage disks that ensure that sensitive data is not lost or compromised intentionally or unintentionally by unauthorized removal from secure areas in which the data is normally used. While non-magnetic marker systems are available for surveillance purposes and would have the advantage of not interfering with the magnetic media of data disks, they are generally not physically suited to the miniature size and slim geometries of conventional 5.25" and 3.5" data disks, of the magnetic, magneto-optical and optical types, that are widely used in personal computers.
One proposal for a surveillance system for personal computer data disks involves the attachment of a low coercivity, high magnetic permeability ferromagnetic marker strip to an inside surface of the jacket in which the disk is housed. This, however, would allow the system to be easily defeated by removing the disk from the jacket and reinserting it into another, non-protected jacket with the data on the disk still intact.
An improved form of surveillance protection for computer data disks is disclosed in copending application Ser. No. 255,882 now U.S. Pat. No. 4,910,625 assigned to the assignee of the present application. As described in this copending application, a band of preferably amorphous ferromagnetic marker material is integrally formed directly on the data disk about the central aperture of the disk and concentric therewith. This has the advantage that the disk itself bears the surveillance marker thus eliminating the ability to defeat the protection by removing the disk from the jacket.
A particular advantage of disks bearing ferromagnetic markers in this manner is that computers used in security areas may be provided with sensors in the disk drive to detect the presence of the marker on a disk loaded into the drive in a manner comparable to that described above. If a security disk with the marker is detected by the sensor, a write enable flag is set in the computer to allow data to be transferred from memory and recorded on the disk. In the absence of a detected marker, the write enable flag is not set and recording of data onto the disk is inhibited thus preventing the unauthorized transfer of security data onto unsecured disks. Such an arrangement is also described in the aforementioned copending application.
In the case of some disk drive systems, as used, for example, with 3.5" microfloppy disks, the disk is provided with a central drive hub, usually comprised entirely of a ferromagnetic metal, which is adapted to be attracted by a magnet on the disk drive spindle to aid in clamping the disk to the drive spindle. Typically, the spindle magnet has a field strength of 50-150 Gauss. The magnetic media on which the data is recorded on the disk is not adversely affected by this field, first because the magnet field is largely concentrated in the vicinity of the metal hub and secondly because the magnetic medium on the disk has a coercivity that is somewhat higher than any residual fringing fields between the magnet and the hub that might extend into the data region on the disk. On the other hand, a surveillance marker of the type described in the aforementioned application, preferably being of a low coercivity amorphous ferromagnetic material, and being located at the periphery of the disk central aperture adjacent the drive spindle, can have its magnetic state controlled or fixed by the fringe field between the spindle magnet and the hub thus preventing the relatively low intensity interrogating field of the drive sensor from detecting the presence of the marker. It is therefore desirable to provide an arrangement that ensures that the marker on a metal hubbed disk can be detected by the sensor in the presence of a magnetic disk clamp on the drive spindle.
Moreover, the metal drive hub in a conventional 3.5" microfloppy disk, which is made of ferromagnetic steel, can be magnetized by the field of the drive spindle magnet with a certain amount of the magnetism being retained in the hub when the disk is removed from the drive. As in the case of the disk drive, the fringe field from the magnetized hub can act to fix the magnetization of the marker on the disk and thereby disable the marker in the presence of a relatively low intensity magnetic field as is normally used in an area surveillance portal detector.
It is therefore an object of the present invention to provide a metal hubbed computer data disk with a magnetic surveillance marker that allows detection of the marker in the presence of a low intensity magnetic interrogation field.
It is another object of the invention to provide a metal hubbed computer data disk with a magnetic marker that allows the detection of the magnetic marker when the disk is positioned on the drive spindle of a disk drive using a magnetic clamping spindle.
It is a further object of the invention to provide a metal hubbed computer data disk with a magnetic surveillance marker that is capable of being detected in a low intensity magnetic interrogation field of a surveillance portal detector.